In total hip arthroplasty, the head and neck of the femur are removed and replaced with a femoral stem prosthesis, along with a matching cup in the acetabulum. Barring complications, a total hip replacement provides a patient with a hip joint that functions much like the natural hip joint, typically for a 20 to 30 year period. However, over time, the implant may wear out or loosen. In such cases, it becomes necessary to replace the original hip joint prosthesis through a revision surgery. Successful revision of a primary total hip prosthesis can be challenging because there is less natural bone to work with, due to the previous removal of the head and neck and neck, and the loss of bone in the area of the primary implant.
For younger patients, it is often desirable to delay total hip arthroplasty in order to preserve bone mass and minimize the risk of requiring a first and possibly a second revision procedure later in life. This can be accomplished by using a resurfacing femoral implant, such as the type disclosed in U.S. Pat. No. 4,123,806 (Amstutz et al.). A resurfacing implant preserves the femoral neck and most of the femoral head. Femoral head resurfacing procedures typically rely upon the placement of a guide pin in the femoral neck. The guide pin protrudes from the femoral head, where it serves as a drive axis for accurate reaming of the surface of the femoral head for receipt of the resurfacing femoral implant. Once the femoral head has been shaped to receive the resurfacing femoral implant, the guide pin is removed. Pin placement theory is based on four primary criteria: (1) in the coronal plane, the pin is centered in the neck in order to prevent notching; (2) in the coronal plane, the pin is either placed centrally in the neck or at a templated angle, such as 140 degrees; (3) in the transverse plane, the pin is centered in the neck; and (4) in the transverse plane, the pin is placed so as to retain neck anteversion, unless the neck is very anteverted.
A guide pin can be placed in the femoral neck without the use of guide instruments. Using external imaging in the coronal and transverse planes, the surgeon identifies the center of the femoral neck, ignoring the location of the femoral head. Diathermy can be used to mark up the center of the femoral neck at about 140 degrees in the coronal plane. The guide wire is then passed into the femoral head.
Placement of the guide pin by eye can provide good results, particularly when performed by a surgeon skilled in hip resurfacing. However, placement by eye can also lead to inaccuracy or inconsistency in pin placement, and therefore to less than optimal results, particularly when practiced by an inexperienced surgeon. Therefore, efforts have been made to provide instruments for use in accurately positioning the guide pin in the femoral neck. Examples include U.S. Pat. No. 6,156,069 (Amstutz et al.) and WO 2005/051,209 A1 (Sheldon et al.), both of which are commonly assigned and are incorporated herein by reference.
Despite the advantages provided by the instruments disclosed in U.S. Pat. No. 6,156,069 (Amstutz et al.) and WO 2005/051,209, further improvements in the adjustability of positioning of pins and guide wires to improve the accurate placement of femoral prostheses are desirable. Applicants have made previous use of the femoral neck clamps and pin guides of the type shown in FIGS. 7-8. The femoral neck clamp reliably guides the guide pin through the center of the femoral neck and minimizes the risk of notching during reaming of the femoral head. However, the femoral neck clamp does not guide varus/valgus or anteversion. Varus/valgus or anteversion can be obtained by eye by alignment with the neck or through the use of a protractor. Applicants have also experimented with the use of a nonadjustable goniometer to achieve accurate varus/valgus and anteversion. In doing so, applicants discovered the desireability of providing an adjustable goniometer and instrument assembly having the features and advantages described herein.